Bone is a connective tissue derived from
mesoderm. Bone consists of minerals - mostly calcium
hydroxyapatite which is composed of calcium and
phosphate - laid into a framework of connective
tissue, like bricks laid between the wooden framework
of a house. There are three types of bone cells:
osteoblasts, osteocytes and osteoclasts. Osteoblasts
are responsible for first making the framework and
then laying in the mineral. Once the bone is
constructed, osteoblasts mature into osteocytes which
are then responsible for nourishing the bone
surrounding them. Osteoclasts are bone sculptors; they
remove bone to help remodel and shape the bones as
they grow.

New bone is first laid down in a rather random
fashion; consequently, it is referred to as woven
bone. Woven bone is then remodeled into lamellar
bone. As the name implies, lamellar bone is laid in
concentric layers, like the rings of a tree. Lamellar
bone can be either compact or spongy. A cross-section
of a long bone, such as the femur, shows a good
example of both compact bone forming the perimeter of
the femur and spongy bone in the form of trabeculae
and spicules filling its inside cavity. The lamellae
of compact bone are fairly uniform in width and highly
organized. Canals carrying blood vessels and nerves
run both lengthwise and horizontally through the
compact bone. The lamellae of spongy bone, on the
other hand, is laid down in varying shapes and
thicknesses. Compact bone on the outside of the long
bone provides strength, support, and protection. The
trabeculae and spicules within the cavity of the bone
allow the bone to be lightweight and provide a space
in which bone marrow can grow.